A known valve arrangement of this type is used for connecting compressed-air pulses to beg-shaped filter elements in a filtering plant for cleaning polluted gases. The compressed-air pulses are connected to the interior of the bag-shaped filter elements for cleaning these elements by striking or shaking loose dust which, in the cleaning of polluted gases, has attached to the walls of the filter elements. The valve arrangement is then mounted on a compressed-air tank. The wall of the compressed-air tank is formed with a through hole, via which the valve member of the main valve, when in closing position, extends into sealing engagement against a valve seat arranged in the compressed-air tank at one end of a pressure tube. This tube extends out from the compressed-air tank in order, when the valve member is temporarily moved to its inner opening position, to connect a compressed-air pulse to the interior of the filter element.
In this prior-art valve arrangement, the main valve has a circular membrane clamped between the valve housing and the wall of the compressed-air tank an covering the hole in the tank wall. The valve member is fixed on the outside of the membrane. It is surrounded by an annular membrane portion which has overflow through holes forming the inlet of the valve chamber and providing a connection between the compressed-air tank and the valve chamber. In the known valve arrangement, the outlet of the valve chamber consists of an annular gap which opens at the outside of the valve housing. This gap is closed when the control valve is in its closed position, and open when the control valve is in its open position.
When the valve member of the main valve is in its closing position, it is maintained in engagement against the valve seat by the pressure prevailing in the valve chamber, which via the overflow holes in the membrane communicates with the compressed-air tank. Thus, the pressure in the valve chamber is the same as in the compressed-air tank, but since this pressure acts on a larger surface than does the pressure in the compressed-air tank, the membrane will be pressed out from the valve chamber, such that the valve member is urged towards its closing position.
When the control valve is opened, the air in the valve chamber flows out through the annular gap, and the pressure in the valve chamber drops, such that the pressure in the compressed-air tank urges the membrane into the valve chamber so as to open the main valve by moving the valve member to its inner opening position. When the control valve is closed shortly afterwards, the pressure in the valve chamber is again built up via the overflow holes in the membrane, whereby the main valve is closed by the valve member moving to its closing position.
In the known valve arrangement, the valve member is made of plastic and fixed to the outside of the membrane by means of a fixing bolt which extends through a plastic washer arranged at the inside of the membrane, through the membrane in its center and through the valve member, and on which a nut is screwed. Thus, the main valve is quite complex in that it consists of many parts. Moreover, the fixing bolt and the nut are liable to corrode in the environment where the filter element and, hence, the valve arrangement are used. As a result of corrosion, the fixing bolt, the valve member and the plastic washer will easily come loose when subjected to vibrations. Also, the membrane is difficult to clamp between the valve housing and the wall of the compressed-air tank, since this mounting must be accurately adjusted to the stroke length of the membrane, that is the distance up to the valve seat on the pressure tube. The known valve arrangement also suffers from the drawback of the membrane easily breaking at the overflow holes.